The 27 kD protein previously observed during the study of thyroxine-binding globulin (TBG) synthesis by human hepatocarcinoma cells has been shown to be distinct immunochemically from TBG. Its biosynthesis is directed by messenger RNA distinct from TBG mRNA. The 27 kDa protein is able to bind T4 and thus represent a previously unrecognized thyroid hormone transport protein of human plasma.